Random copolymers play an important role in a range of soft materials applications and biological phenomena. An individual monomer is typically a single chemical unit whose length is comparable to or less than a Kuhn length, resulting in a monomer segment that is structurally rigid at length scales of a segregated domain. Previous work on random copolymer phase segregation addresses the impact of correlations between the chemical identities along the chains for flexible polymers. In these works, a single monomer unit is effectively a large polymer block that behaves as a random walk without conformational correlation associated with semiflexibility. In our work, we develop a model of semiflexible random copolymers using the wormlike chain model to capture conformational correlation of the polymer chains. To address the thermodynamics of microphase segregation and the structure of the segregated domains, we develop a random phase approximation up to quartic order in density fluctuations that leverages our exact results for the statistical behavior of the wormlike chain model. In this work, we focus on the quadratic-order expansion of the free energy, which provides the mean-field spinodal of the homogeneous phase. We explore the impact of conformational and chemical correlations on the formation of inhomogeneous microphases at the spinodal point. We show that the onset of phase segregation and the correlation length of domains are extremely sensitive to chain rigidity. ■ INTRODUCTIONBlock copolymers have attracted attention in numerous applications because of their ability to self-assemble into nanostructured morphologies. 1−3 Extensive theoretical and experimental studies have predicted and characterized the ordered morphologies of block copolymers with well-defined chemical architecture such as diblock and triblock copolymers. 4−7 In contrast, the phase behavior of multiblock copolymers is less well understood. In particular, theoretical, computational, and experimental studies on random copolymers, whose monomer chemical identities are stochastically arranged along the polymers, are far less prevalent. 8−11 Commercial products such as high impact polystyrene (HIPS), styrene−butadiene rubber (SBR), and Nafion are blends of macromolecules with randomly distributed chemical repeat units. 12,13 In these materials, the mesoscale morphology greatly influences their mechanical and transport properties. Recently, there has been interest in making new soft materials by introducing chemical stochasticity into block copolymers, such as tapered diblock copolymers 14 and gradient copolymers, 15−17 due to their ability to relieve packing frustration during self-assembly. Biology also needs to overcome random variability to achieve self-assembly in macromolecular environments in scenarios such as protein folding and chromatin condensation. 18−20 Despite the abundance of random copolymers in various industrial applications and biological systems, the understanding of their phase behavior and structure− property relationship ...

Alkaline exchange membranes (AEMs) are a promising class of polyelectrolytes whose alkaline operating environment enables the use of non-precious metal catalysts in low-temperature fuel cells. However, their poor ionic conductivities, which are often an order of magnitude lower than traditional acidic membranes (e.g., Nafion), have limited their practicality. The performance problem can partially be ascribed to the poorly-defined morphologies of typical random copolymer AEMs, leading to tortuous ion transport pathways. Here, we show the ability to form nanoscale (5 to 10 nm diameter) anion transport channels by grafting hydrophilic poly(ethylene glycol) side-chains along a model benzyltrimethylammonium polysulfone-based AEM. Concomitant with the structure formation is a 100% increase in the IEC-normalized hydroxide conductivity from 20.2 mS g cm À1 mmol À1 to 40.3 mS g cm À1 mmol À1 as well as a 50% increase in the peak power density from 118 mW cm À2 to 180 mW cm À2 when incorporated into a fuel cell.

There is a delicate balance between ion exchange capacity (IEC), conductivity, and dimensional stability in anion exchange membranes as higher charge content can lead to increased water uptake, causing excessive swelling and charge dilution. Using highly-charged benzyltrimethylammonium polysulfone (IEC=2.99 mEq g(-1) ) as a benchmark (which ruptured in water even at room temperature), we report the ability to dramatically decrease water uptake using a semi-interpenetrating network wherein we reinforced the linear polyelectrolyte with a crosslinked poly(styrene-co-divinylbenzene) network. These membranes show enhanced dimensional stability as a result of lower water uptake (75 % vs. 301 % at 25 °C) while maintaining excellent hydroxide conductivity (up to 50 mS cm(-1) at 25 °C). These improvements produced an enhanced alkaline fuel cell capable of generating 236 mW cm(-2) peak power density at 80 °C. This method is easily adaptable and can be a viable strategy for stabilizing existing systems.

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